Add test for releasing connections back to the server

design.md

@@ -298,7 +298,16 @@ This write-side component is designed to integrate with:
 - **Configuration**: All configuration is TOML-based using `config.toml` (see `config.md`)
 - **Testing Strategy**: Run unit tests, benchmarks, and debug tools before submitting changes
 - **Build System**: CMake generates gperf hash tables at build time; always use ninja
-- **Test Synchronization**: NEVER use sleep() for test synchronization - it makes tests slow and flaky. Use proper synchronization primitives like std::latch (C++20), condition variables, or promises/futures instead
+- **Test Synchronization**:
+  - **ABSOLUTELY NEVER use sleep(), std::this_thread::sleep_for(), or any timeout-based waiting in tests**
+  - **NEVER use condition_variable.wait_for() or other timeout variants**
+  - Use deterministic synchronization only:
+    - **Blocking I/O** (blocking read/write calls that naturally wait)
+    - **condition_variable.wait()** with no timeout (waits indefinitely until condition is met)
+    - **std::latch, std::barrier, futures/promises** for coordination
+    - **RAII guards and resource management** for cleanup
+  - Tests should either pass (correct) or hang forever (indicates real bug to investigate)
+  - No timeouts, no flaky behavior, no false positives/negatives
 
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